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ziocalepino 15th October 2012 07:58 AM

A strange sensitivity case
Hi everybody. Yesterday I've tried two kind of drivers, Lowther PM6C vs. Voxativ Ac-2.8 On the paper Voxativ have a 4-5 dB greater sensitivity respect Lowther, but mounted on the same cabinet (Acousta 116), with the same system, CD track and amplifier gain, they sound LESS LOUD. Can anybody explain me this strange result?
Many thanks in advance

Scottmoose 15th October 2012 08:24 AM

Across the entire frequency range, or just in terms of box output? Can you provide some links to the Voxativ technical data, I can't find that unit on their site & I don't have time to traul the 'net in search of it.

ziocalepino 15th October 2012 08:26 AM

For all frequencies

Scottmoose 15th October 2012 08:53 AM

Data on the driver?

ziocalepino 15th October 2012 09:09 AM

Taken from the net:

Technical Data :
Frequency response : 20 - 20.000 Hz
Efficiency : 99 dB / 1W / 1 m
Capacity : 50 W nom / 100 W mus
Impedance [R] : 10,5 Ohm
Complex impedance [Z] : 16,0 Ohm
Resonance frequency : 43,0 Hz
Mechanical Q-factor Qms : 3,31
Electrical Q-factor Qes : 0,40
Q-factor Qts : 0,36
Equivalent volume [VAS] : 66,0 Ltr
Xmss : 3 mm
Depth : 7,5 cm
Install diameter : 7,5" / 19,0 cm
Weight : 4,8 kg

Lowther PM6C

DC Resistance of Voice Coil Re 7.4 ohms Free Air Resonance Fo 45 Hz Emission Piston Area Sd 0.021M˛ Equivalent Volume Suspension Vas 56.046 Ltrs Suspension Compliance Cms 894.981 u M/N Mass of Cone Mmd 11.0 g Mass of Moving Parts Mms 12.75 g Force Factor Bl 7.246 T-M Mechanical Q Factor Qms 4.788 Electrical Q Factor Qes 0.510 Total Q Factor Qts 0.461 Voice Coil Inductance @ 1 kHz Le 87.723 uH Reference Efficiency No 1.11% Sound Pressure Level SPL 92.473 Db

Scottmoose 15th October 2012 09:29 AM

The Voxativ data set isn't particularly comprehensive, but from what is available, sensitivity would appear to be about 93dB 1m/w or 91.9dB 1m 2.83v, not 99dB. Lowther's published data is much more consistent for the stated sensitivity value.

So the data doesn't support any definitive conclusions, other than that the Voxativ SPL figure does not appear to agree with the rest of the technical information they publish. There are a number of possible reasons for this; it can only really be a matter of speculation though unless both units can be measured with the same equipment to ensure consistent methodology. There could be a measurement, or a typographical error. They could be taking the SPL value as the maximum it attains at one point rather than a derivation from its electromechanical properties. And so on & so forth. Or it could be something completely different. Which brings us to why the Voxativ should sound quieter than the Lowther when they appear in practice to be much of a muchness in terms of outright sensitivity. Again, we can only speculate without more data. Here's one possibility: most Lowther units have significant peaking above ~5KHz, so if the Voxativ has an inherently flatter FR, that might be what is happening. Alternatively, the technical data from one or both manufacturers might be significantly out from what is actually the case; if that is so, then the Lowther may indeed simply have greater efficiency (& thereby sensitivity) than the Voxativ. As I say, without actually measuring both drivers on the same gear to find out what is really happening, it's not going to be possible to obtain a conclusive answer.

ziocalepino 15th October 2012 09:48 AM

A great, almost exhaustive answer. Many thanks!

ingvar ahlberg 15th October 2012 10:28 AM

Also, the Voxativ is a 16ohm driver, there´s 3dB gone on same "volume setting"
if it wasn´t measured output voltage refered to as same gain.

ziocalepino 15th October 2012 10:55 AM

To tell the truth PM6C also are 15 ohm... mine are.

ingvar ahlberg 15th October 2012 11:08 AM

Ok, the Re 0f 7,5 looked closer to a nominal 8 than a 16ohm unit which mostly have an Re over 10 ohm.

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